Chipset doesn't matter. If it did, you'd only be able to use those cards on CF/SLI boards. Thats the point to the "x2" cards, it doesn't matter what chipset you use.
The answer to this is whether the GTX295 is worth the extra $$$. Both cards are very fast, either can handle 19x10 with ease. Unless you are on a huge 30" 25x16 screen, the performance difference between these two won't really be felt. Gkay09 brings up a good point. Depending on your resolution, neither of these cards might be needed. If you are like most and have a 16x10 22" display, I wouldn't get either of these cards.